AMOLED (Active Matrix Organic Light Emitting Diode) is used more and more widely. The pixel display elements of the AMOLED are Organic Light Emitting Diodes (OLEDs). The driving thin film transistors generates a driving current in a saturation status, and then the driving current drives OLED to emit light, thereby the AMOLED emits light. FIG. 1 shows the schematic structural view of a basic pixel driving circuit in the prior art. The existing basic pixel driving circuit uses a 2T1C circuit comprising two thin film transistors and a storage capacitor, as shown in FIG. 1.
However, because of the poor uniformity in the threshold voltages Vth of the driving transistors DTFT in the conventional low-temperature polysilicon process technology, and the drift in use, when the same data voltage Vdata is input to the driving transistors DTFT, different driving currents will be generated due to the different threshold voltages of the driving transistors DTFT, resulting in poor uniformity of AMOLED luminance.
To overcome above problems, an AMOLED pixel driving circuit with threshold voltage compensation has been proposed by those skilled in the art. FIG. 2 shows the schematic structural view of a pixel driving circuit with threshold voltage compensation in the prior art. FIG. 3 is an equivalent circuit of that shown in FIG. 2 in discharging stage. The circuit shown in FIGS. 2 and 3 is a 6T1C circuit. In a charging stage, the driving transistor DTFT is cut off from high level VDD and low level VSS by controlling the switch transistors, while one terminal of the storage capacitor C is connected with a voltage input terminal, and the other terminal of the storage capacitor C is connected with the data line. An initial voltage Vini is provided by the voltage input terminal, the data voltage Vdata is provided by the data line. The storage capacitor C is charged by both the voltage input terminal and the data line, such that a voltage Vini-Vdata is generated across the storage capacitor. With reference to FIG. 3, in a discharging stage, the driving transistor DTFT is cut off from high level VDD, low level VSS, data voltage and the initial voltage Vini by controlling the switch transistors, while the switch transistor S1 connecting with the gate and the drain of the driving transistor DTFT is kept turned on, such that the storage capacitor C connecting with the gate and the drain of DTFT is discharged through the driving transistor DTFT. When Vg=Vdata+Vth, the driving transistor DTFT is in a sub-threshold ON state, the discharge is over and the voltage across the storage capacitor is Vth, wherein Vg is the gate voltage of the driving transistor. Vdata is the data signal voltage, and Vth is the threshold voltage of the driving transistor DTFT. In a stage of emitting light to display, Vg=Vdata+Vth, Vss=VSS, Vgs=Vdata+Vth, wherein Vg is the gate voltage of the driving transistor DTFT, Vss is the source voltage of the driving transistor, Vgs is the gate-source voltage of the driving transistor DTFT. Since I=K*(Vgs−Vth)2, I=K*Vdata2, wherein I is the driving current in the driving circuit, therefore the driving current is independent of the threshold voltage, and the threshold compensation is achieved.
Above circuit, however, may implement the threshold compensation only when the driving transistor DTFT is an enhancement type transistor. When the driving transistor DTFT is of depletion type, the threshold voltage Vth of a depletion type transistor is negative. In the discharging stage, the driving transistor will stop discharging when Vg=Vdata, wherein Vg is the gate voltage of the driving transistor DTFT; because the voltages of the gate, source and drain of the driving transistor DTFT are equal to Vdata at this time, that is, the source-drain voltage Vsd=0, such that the discharge is over. The voltage across the capacitor is 0 rather than Vth, thus the threshold compensation is unavailable in the AMOLED pixel driving circuit.